In what is generally accepted by those skilled in the papermaking art as a conventional process for manufacturing paper webs for use as sanitary tissue and towel products, the web is subjected during the dewatering process to one or more pressing operations over the entire surface of the web. In one such conventional papermaking process, the web is formed on a Fourdrinier wire and then is transferred to a pick up felt. The pick up felt and the paper web is passed through a nip formed by a pressure roll and the surface of a heated Yankee Dryer cylinder. The felt has a relatively smooth surface so that as the felt and paper web travel through the nip, pressure is applied uniformly over the entire surface of the moist paper web for the purposes of squeezing water out of the paper web into the felt, developing tensile strength in the web, smoothing the surface of the web and adhering the surface of the web to the surface of the drying cylinder. The web is then creped off of the surface of the drying cylinder.
If the conventional process for manufacturing a paper web represents one end of the spectrum, then the process described in U.S. Pat. No. 3,301,746 - Sanford, et al is representative of the other end of the spectrum. In the process described by Sanford, et al, a paper web is laid down on a forming wire and then is predryed in a hot air through dryer; this unpressed, pre-dryed, to at least 30% dry, but preferably 40% dry, web is then deposited onto the surface of a creping cylinder, is imprinted with the knuckle pattern of an imprinting fabric and then is creped off of the surface of the drying cylinder. Sanford, et al found that the resultant web when compared to a creped web made in accordance with a conventional papermaking process has increased softness, bulk and absorbency characteristics while at the same time having substantially the same tensile strength as the creped conventional web.
Although the Sanford, et al process is capable of making extremely soft, bulky and absorbent paper products when compared to paper products made using a conventional papermaking process, the use of a hot air through dryer is more complex, more expensive and less efficient than the Yankee Dryer used in a conventional papermaking machine.
Others in the prior art have considered it desirable to produce paper products having the softness, bulk and absorbency characteristics between those obtained from the conventional papermaking process and those obtained from the Sanford, et al process. It has been proposed in the prior art that the concept of differentially pressing the web in spots on a dryer as disclosed by Sanford, et al could be applied to the web being deposited onto the surface of the Yankee Dryer cylinder in a conventional papermaking machine. For example, U.S. Pat. No. 3,537,954 - Justus discloses several embodiments of paper machines in which a paper web is differentially pressed in spots while on the surface of a Yankee Dryer cylinder. FIG. 4 of Justus depicts a conventional papermaking machine configuration and is representative of the teaching of that patent. At column 4, lines 41-68, the papermaking machine of FIG. 4 is described as one in which the web is differentially pressed onto the surface of the drying cylinder by means of a special pick up felt. In describing the special pick up felt, Justus states that the design of the felt will be obvious to those skilled in the felt making art and that its weaving pattern may for example include hard twisted yarns or monofilament fillers running in a cross machine direction along the outer surface of the felt. These hard yarns or fillers should be spaced depending on the desired creping pattern. The hard yarns will cause higher localized pressure areas at the nip so that the creping pattern thus imparted to the web will follow the higher pressure areas to which the web has been previously subjected. The soft and resilient felt material located between the harder yarns will urge the web into intimate contact with the surface of the drying cylinder. That is the extent of the teaching of Justus concerning the type of felt to be used in his process.
Based on Justus' summary of his invention at column 1 lines 68-69 and at column 2 lines 12-13, 20-21 and 34-35, it appears that he is concerned with making sure that the entire surface of the imprinted web adjacent to the dryer makes intimate contact with the surface of the drying cylinder. The fact that the entire surface of the differentially pressed web makes intimate contact with the drying cylinder merely means that, for webs having the same basis weight, the dryer will remove as much water out of the differentially pressed web as it would out of the conventional overall pressed web. We believe that the greater problem was the fact that since the differentially pressed web is pressed in spots on the surface of the drying cylinder, less water was expelled or squeezed out of the web as compared to the conventional papermaking process in which the entire surface of the web is pressed. Two commercially unacceptable ways of removing this additional water is to have the sheet remain on the dryer for a longer period of time by slowing down the speed of the dryer or to increase the heat applied to the dryer while the dryer operates at the same dryer speed as in the equivalent conventional process.
It is an object of our invention to provide an improved conventional papermaking machine in which the dewatering felt is replaced with a dewatering and imprinting felt that differentially presses a paper web onto the surface of a drying cylinder. It is a further object of our invention that when our improved papermaking machine makes a paper web having substantially the same basis weight and tensile strength as paper made on a similarly configured conventional papermaking machine, and operates at substantially the same speed as the conventional papermaking machine, the resulting paper web will have increased bulk, softness and absorbency than the paper made on the conventional papermaking machine.